The present apparatus and method embodiments relate to purification of gas, such as carbon dioxide, for use in chemical, pharmaceutical and beverage use.
Known methods for carbon dioxide (CO2) recovery utilize a series of absorption and adsorption steps to remove impurities from a feed gas. Feed gas streams are normally waste streams from processes where CO2 is a significant by-product. The feed gas stream also contains impurities that are undesirable for the final product and must be removed through purification processes. Feed gas streams originate from manufacturing activities that include for example ammonia production, fertilizer production, fermentation and combustion processes.
Feed gas streams include impurities that are unacceptable for use with a subset of industries that use CO2, including pharmaceutical production, carbonation of beverages and processing of food. Such impurities include sulfur compounds, volatile organic compounds such as aromatic and aliphatic hydrocarbons, odorous compounds (including but not limited to hydrogen sulfide (H2S), carbonyl sulfide (COS), (dimethyl sulfide (DMS), mercaptans), heavy metals, particulate matters and nitrogen oxides, among others. The species and concentration of the impurities are a function of the process that produces the feed gas. For example, fermentation processes produce alcohol, a volatile hydrocarbon. Combustion processes are likely to produce sulfur in the feed gas originating from the fuel used for combustion. These impurities must be reduced to concentrations that are acceptable for the end use; especially for beverage, food and pharmaceutical applications where regulatory and voluntary guidelines specify maximum allowable concentrations for impurities. Certain of these impurities even when present in amounts below regulatory, mandatory or voluntary guidelines are sometimes not desirable at all, such as impurities that impart taste in beverages or where CO2 is used in direct contact with pharmaceutical drug products. There also are self-imposed preferences by customers in sourcing CO2 for processing and accepting for use. Therefore, producers are forced to continue to drive improved quality of CO2.
Conventional CO2 production facilities use a series of steps to concentrate and purify CO2 product. All of the common impurities require a specific treatment in order to be removed from the feed gas stream. Some impurities are very soluble in water and can be removed using absorption with either water or a caustic solution in a wet scrubber. Other impurities can be removed using adsorption, wherein the impurity is bound to a surface or a chemical component on the surface and or held in pores of adsorbent material. Some of these processes are reversible by using either heat or pressure swings during a regeneration step. Other process materials cannot be easily regenerated and therefore the adsorbent must be sacrificed when it has reached its saturation limit. This creates a burden on a CO2 production facility because of the expense of replacing material and the opportunity cost due to downtime required to service the material beds. Sacrificial beds are also very sensitive to the incoming concentration of impurities, since they have a finite capacity for retaining the impurities.
A known CO2 purification and production process is shown in FIG. 1. The stages of said process include providing the CO2 feed gas from a production process; a pre-compression cleanup stage, wherein the primary cleaning of CO2 feed gas is accomplished by water washing, aqueous alkaline scrubbing and oxidative scrubbing using potassium permanganate (KMnO4) at low pressure. Depending upon the source for the feed gas and impurities in the gas, all three types of scrubbing may not be required.
A post-compression cleanup stage for the impurities is mainly by adsorption processes, the impurities handled being at a much lower concentration than in the pre-compression stage.
FIG. 2 shows an arrangement of a known feed gas scrubbing occurring in the pre-compression cleanup stage of FIG. 1.
Absorption processes, such as that of FIG. 1, provide a less expensive way of removing a bulk of soluble impurities such as alcohol, aldehydes, sulfur compounds, etc. Using chemical reagents such as potassium permanganate (KMnO4), sparingly soluble impurities such as nitrogen oxides (NOx) are oxidized and removed. However, the effectiveness of absorption processes is sometimes limited due to low solubility of impurities and low allowable purge of spent scrubbing medium. In order to handle increased impurity level, robust and reliable performance of the pre-compression stage is vital in maintaining quality of the product and effectiveness of the downstream purification stage.
As shown in FIG. 1, in the pre-compression stage, primary cleaning of CO2 feed gas is done by wet scrubbing (absorption processes). Primary cleaning by wet scrubbing is achieved by use of one or more scrubbing towers with use of one or more reagents.
Generally, CO2 containing feed gas stream is first contacted with water in a co-current or countercurrent fashion for direct contact. The water acts as a scrubbing agent, dissolves soluble impurities and carries away the particulates in the waste stream. This step requires huge amounts of wash water in a once through mode and generates a large quantity liquid effluent stream that must be processed. If the water wash stream is recirculated in a closed loop, concentration of impurities gradually builds up and removal efficiency deteriorates. When the scrubber water has absorbed all the impurities it is said to be saturated. Saturated water must be drained from the scrubber and replaced with clean water in order for more impurities to be removed. In a recirculating system the water is always partially saturated so a good balance must be made between fresh water make-up and the concentration of impurities in the CO2 exiting the scrubber. Typical impurities that are removed in water washing are acetaldehyde, alcohols, ketones, ammonia and hydrogen chloride (HCl), for example.
For removal of acidic (impurities), caustic/soda based water scrubbers are used downstream of water wash scrubber. Sodium carbonate (Na2CO3) or sodium hydroxide (NaOH) is dosed in an aqueous recirculation medium to maintain slight alkalinity. Acidic impurities such as sulfur dioxide (SO2) and HCl are removed, along with some hydrogen sulfide (H2S) and CO2, by wet alkaline scrubbing to form water soluble compounds.
Potassium permanganate (KMnO4) is a powerful oxidizer and can oxidize a number of impurities to compounds that are soluble or insoluble in the potassium permanganate solution. Amongst many impurities oxidized by scrubbing in KMnO4 scrubbers, removal of nitrogen oxides in particular is unique. Other impurities that are removed include sulfur compounds and some odorous and taste imparting compounds. In order to maintain high efficiency, permanganate scrubbers must operate under alkaline conditions. CO2 in the feed gas stream has the effect of neutralizing the scrubber solution by forming carbonate and bicarbonate. Under neutral conditions, Manganese Dioxide (MnO2) precipitates creating serious operational issues due to fouling of scrubber packing and clogging of the scrubber components.
The effectiveness of permanganate scrubbers is impacted by the incoming NOx concentration. Often, the capacity of CO2 plants must be reduced when the feed gas concentration exceeds the design range. It is also common to require frequent service for permanganate scrubbers when NOx levels exceed the normal range. Some production facilities that experience spikes in nitrogen oxide concentrations in the feed gas must shut down the system ever few hours to remove the old potassium permanganate solution and replace it with new or fresh solution. It is not uncommon to require up to more than one or two shutdowns per day to service permanganate scrubbers. In some cases CO2 producers reduce the plant capacity to increase the length of time the permanganate scrubber can stay in service before it needs to be re-charged.
Post compression cleanup is somewhat of a polishing stage and mainly consists of adsorption processes to further reduce impurities. The most commonly used beds include Zinc Oxide (ZnO), Silica, Alumina and Carbon for removing many different impurities.
In addition, high pressure water washing may also be used to lower soluble impurities. Some configurations include catalytic reactors to convert some hard to treat impurities.
Adsorption beds remove one or a plurality of impurities or component impurities in the feed gas. ZnO, ferrous and ferric oxide beds for removal of H2S, activated carbon is effective in adsorbing impurities like acetaldehyde, aromatic hydrocarbon and other volatile hydrocarbons. Silica beds are effective in removing water, oxygenates such as alcohols. The capacity of an activated carbon bed is a function of the impurity's species and concentration. The capacity of a given bed is also limited by the amount of absorbent it holds, therefore it is used as a polishing bed. It is advantageous to remove as much of these impurities as possible in a pre-compression cleanup stage before the feed gas reaches the polishing bed.
Even beds that can be regenerated are affected by the concentration of impurities, because the operating cycle is affected by the amount of time it takes for the bed to reach capacity. Regeneration cycles tend to add cost to the process due to their need for heating energy or pressurization. For example, carbon adsorption beds require a large amount of steam to raise the temperature of the bed to the temperature needed to remove the adsorbed impurities. Often CO2 producers are adversely affected by “spikes in impurities” that are in excess of the design capacity of polishing beds designed to remove them.
Therefore, CO2 producers will want to remove as much of the impurities as possible in precompression cleanup stage upstream of the adsorption and polishing steps. A KMnO4 based oxidative scrubber is generally placed downstream of water wash scrubbers or alkaline scrubber. Improving reliability in oxidative scrubbing will reduce not only impurities due to oxidative chemistry, but also provide an additional stage for removal of soluble impurities.
As mentioned above, for oxidative scrubbing, KMnO4 solution is used in an aqueous scrubber. KMnO4 is a strong oxidant. However, the oxidation occurs in the liquid phase and has the following issues. Impurities from the gas phase are required to first dissolve in order to react with KMnO4. Some of these impurities have very poor solubility and require large gas liquid contact to effectively transport across the gas-liquid interface. Most scrubbers do not provide adequate scrubbing when impurities spike during production. CO2 in the feed gas neutralizes alkalinity of KMnO4 solution which significantly retards oxidation rates and removal efficiencies. Contaminants such as H2S, DMS COS, mercaptans impart objectionable taste and odor, even at very low concentrations. Therefore, inefficiencies in oxidative scrubbing are not acceptable.
Precipitation of manganese oxides fouls packing in absorption columns which reduces gas-liquid contacting surface area, thereby rendering the columns to be less efficient and not as cost-effective.